Method for the production of cross-linkable acrylate contact adhesive materials

ABSTRACT

Process for preparing crosslinked pressure ensitive adhesives, in which a plyacrylate is prepared by the free radical copolymericzation of a monomer mixture , the polyacrylate is then concentrated and the concentrated polyacrylate is reacted with further monomers having functional groups and subsequently crosslinking.

This is a 371 of PCT/EP01/06770 filed 15 Jun. 2001 (international filingdate).

The invention relates to a process for preparing polyacrylates which arefunctionalized with double bonds and have pressure-sensitively adhesiveproperties, whose cohesion is increased by radiation-inducedcrosslinking, and to an adhesive tape provided with this polyacrylatepressure sensitive adhesive.

BACKGROUND OF THE INVENTION

Hotmelt pressure sensitive adhesives (hot melt PSAs) are compounds whichcombine the properties of hotmelt adhesives with those of pressuresensitive adhesives. Hotmelt PSAs melt at elevated temperatures and coolto form a permanently tacky film which flows adhesively on contact witha substrate. In combination with various substrates, such as paper,fabric, metal, and polymer films, for example, it is possible to producea large number of different products, particularly pressure sensitiveadhesive tapes and also labels. These pressure sensitive adhesiveproducts have a broad field of application in the automobile industry,e.g., for fastening or for sealing, or in the pharmaceutical industry,for active substance patches, for example.

The typical coating temperature for hotmelt PSAs lies between 80 and180° C. In order to minimize the coating temperature, the molecularweight of the hotmelt PSA to be applied should be as low as possible. Onthe other hand, the PSA must also possess a certain level of cohesion,so that the PSA tape does not slip from the substrate in use. In orderto increase the cohesion, in turn, a high molecular weight is essential.

In order to solve this problem polymers have been developed whichpossess a relatively low molecular weight but contain double bonds alongthe side chains. These polymers, such as polyester acrylates orpolyurethane acrylates, for example, can be crosslinked efficiently viathe double bonds using UV or ionizing radiation, but have only limitedadhesive properties.

In acrylic PSAs, crosslinking is promoted by adding polyfunctionalacrylates and/or methacrylates prior to crosslinking, which raise thecrosslinking reactivity and hence also increase the cohesion, but whichreact only by way of a two-stage mechanism during irradiation(attachment to the polymer and then crosslinking reaction by way of theacrylate double bond which is still free) and therefore have a lowcrosslinking efficiency.

The principle of functionalizing double bonds by copolymerization cannotbe employed analogously for acrylic PSAs, since in that case thecorresponding polyacrylates are prepared by free radical polymerization.All of the double bonds here are reacted in the polymerization process,or instances of gelling occur during polymerization. One example of thiswas depicted by Pastor [U.S. Pat. No. 4,234,662 A], who used allylacrylate or allyl methacrylate for the polymerization. A centralproblem, however, lies in the copolymerization of these compounds, whichgenerally gel during the free radical polymerization process. Moreover,owing to the relatively low reactivity of the allyl groups in respect ofa crosslinking reaction, drastic experimental conditions are necessary,in particular high temperatures or a long period of irradiation. Forapplication as a crosslinked PSA, therefore, the allyl-modified acrylicpolymers are not very suitable.

Another possibility for functionalization of double bonds exists byvirtue of polymer-analogous reactions.

Generally speaking, polymer-analogous reactions can be conducted insolution or from the melt. EP 0 608 981 B1 likewise refers to thegelling problem with double bonds. This is assisted by diverse furtherpolymer-analogous reactions. Accordingly, polyacrylates with carboxylicacid, hydroxyl, epoxide, and amine groups can be reacted in apolymer-analogous reaction with compounds containing double bonds; inthis regard see U.S. Pat. No. 4,665,106 A. Owing to the low thermalstability of the components involved, however, it has not been possibleto apply this reaction to hotmelts. Moreover, operating conditions weredisadvantageous owing to the fact that in order to avoid gelling it wasnecessary to add large amounts of regulator to the polyacrylate.

For acrylic hotmelts, therefore, U.S. Pat. No. 5,536,759 A describes thereaction of polyacrylates containing hydroxyl or carboxylic acid groupswith 1-(1-isocyanato-1-methylethyl)-3-(1-methylethenyl)benzene (m-TMI)in solution with subsequent hotmelt processing. It is in contrast tothis that the pros and cons of the individual processes are described(Chemie Ingenieur Technik (70), 1998, pp. 560-566); “Polymer-analogousreactions in the melt permit two processes which otherwise proceedseparately from one another. First of all, the reaction takes place;since the reaction medium is the melt, shaping by extrusion can becommenced during the reaction. In this way, no additional reactionvessel and no work-up at all are necessary. Nevertheless, the absence ofthe solvent complicates the course of the reaction in a variety ofrespects; for example, by the heterogeneity of the reaction mixture andthe relatively slow diffusion of the reactants into one another”.

Accordingly, the process described in EP 0 608 981 B1 displays thefundamental disadvantages of a polymer-analogous reaction in solution.What would be desirable, therefore, would be a process for acrylic PSAswhich allows polymer-analogous reactions in the melt.

A central problem lies in the slow diffusion of the reactants. Thisproblem can be solved only by raising the reaction temperatures, whichimproves the reactivity of the individual components with one another.For acrylic PSAs, however, there are natural limits on this.

For polymer-analogous reactions in the melt, therefore, the materialsused are generally thermoplastics, which are processed andfunctionalized at high temperatures. For example, polystyrene-maleicanhydride thermo-plastics are reacted at temperatures of 180-200° C.[Chemie Ingenieur Technik (70), 1998, pp. 560-566 and Chemie IngenieurTechnik (71), 1999, pp. 1418-1421]. Additionally, polyesters are reactedwith maleic anhydride in the melt [Journal of Polymer Science: Part A:Polymer Chemistry, Vol. 37, 1693-1702 (1999)]. Both processes, however,are unsuitable for the functionalization for acrylic PSAs with doublebonds. Journal of Polymer Science: Part A: Polymer Chemistry, Vol. 37,1603-1702 (1999) discloses functionalization by radical grafting, butthis cannot be used for functionalization with double bonds on accountof the fact that the vinyl compounds would immediately be polymerizedand so would no longer be available for subsequent crosslinking on thebacking. The prior art uses polymers [Chemie Ingenieur Technik (70),1998, pp 560-566] whose glass transition temperatures are too high forPSAs and, if applied analogously to acrylic PSAs, would exhibitexcessively high reaction temperatures (at the high temperaturesemployed, severe discoloration of the polymer already occurs, owing toreactions by, for example, thermally decomposing initiators which haveremained after the polymerization process or by the decomposition ofindividual copolymers, such as tert-butyl acrylate, for example, atabove 160° C., and also possess very high fractions of copolymerizedmaleic anhydride, which places the glass transition temperature veryhigh. The process described in U.S. Pat. No. 5,536,759 A for reactingpolyacrylates with1-(1-isocyanato-1-methylethyl)-3-(1-methylethenyl)benzene (m-TMI) insolution can also not be applied analogously for the processes describedabove, since in addition to the high toxicity of the isocyanates thecrosslinking reactivity after coating would be too low.

A serious and general disadvantage of all of the processes described sofar lies in the low crosslinking reactivity after coating. Vinylcompounds have a low reactivity toward the radicals that are generatedfor crosslinking, with the consequence that crosslinking is incompleteand not very effective. Competing reactions which do not lead to thedesired crosslinking, such as saturation of the radicals produced byatmospheric oxygen or added tackifier resins, predominate. The poorcontrollability of crosslinking may therefore be very problematic, forexample, for the aging behavior of the PSA, since unless all of thedouble bonds are consumed by reaction during crosslinking, the PSA willhave a potential for post-crosslinking on prolonged storage and also,under the influence of ultraviolet light or oxygen and/or ozone, willreact and undergo a marked loss of bond strength.

There is therefore a need for compounds which can be reacted veryquickly and without gelling in a polymer-analogous reaction and for aprocess operation which allows gel-free processing and coating on abacking, with the aim of obtaining new kinds of acrylic PSA tapesfunctionalized by reactive double bonds, which can be crosslinked withhigh reactivity by actinic radiation.

SUMMARY OF THE INVENTION

It is an object of the invention to provide a process for preparingpressure sensitive adhesives on an acrylic basis which have aviscoelastic behavior at room temperature and which do not show thedisadvantages of the prior art. The aims are to prevent gelling of thePSAs during the hot melt process; in particular, not to lose the thermalstability of the PSA as a result of incorporation of the double bonds;that it should be possible to subsequently coat the acrylic PSA providedwith reactive double bonds from the melt onto a backing without gel; andthat said PSA should be crosslinked with high crosslinking efficiency.

This object is achieved, surprisingly and unforeseeably for the skilledworker, by a process as set out in the main claim. The further claimsrelate to advantageous developments of this process, to the pressuresensitive adhesive prepared thereby, and to a use for said adhesive.

The invention accordingly provides a process for preparing crosslinkedacrylic pressure sensitive adhesives which involves

-   first preparing polyacrylates by free-radical (co)polymerization    from the following monomers:    -   (a) acrylic and methacrylic acid monomers of the following        structure:        -   where R₁H or CH₃        -   and R₂= an alkyl chain with 2-20 carbon atoms    -   with a fraction of 45-99.5% by weight in the monomer mixture,    -   (b) one or more carboxylic anhydrides containing olefinic double        bonds, with a fraction of 0.5-25% by weight in the monomer        mixture, more preferably with a fraction of 1-5% by weight in        the monomer mixture,    -   (c) further olefinically unsaturated monomers possessing        functional groups A, with a fraction of 0-30% by weight in the        monomer mixture,-   concentrating the polymers thus prepared to give a polyacrylate    composition whose solvent content is ≦2% by weight,-   adding further monomers to the polyacrylate composition, these    monomers possessing at least two functional groups B and C, the    groups B being able to enter into polymer-analogous reactions with    the carboxylic anhydrides, and the groups C being crosslinkable    groups,-   a reaction taking place between the functional groups B and the    carboxylic anhydride, which attaches the monomers containing    functional groups B as side chains to the polymers,-   following the reaction between the functional groups B and the    carboxylic anhydride, applying the pressure sensitive adhesive from    the melt to a backing, and-   carrying out crosslinking of the polymers on the backing by means of    high-energy radiation.

The process makes it possible to introduce into the polymers functionalgroups which are available for efficient crosslinking later on undermild conditions, without the functional groups which permit crosslinkingbeing consumed or losing their functionality during the polymerizationprocess.

As a result of the introduction of the compounds containing thefunctional groups B and C after polymerization has already taken place,the functional groups C which are reactive for crosslinking retain theirreactivity even following incorporation into the polymer chains.Functional groups which possess a high efficiency for the crosslinkingreaction can be introduced into the polymers in this way, even though inthe case of a free-radical polymerization these functional groups wouldlose their functionality.

The average molecular weights (weight average M_(w)) of the PSAs whichform in the course of the free-radical polymerization are chosen suchthat they are situated within a range which is customary forpolyacrylate compositions, i.e., between 100000 and 2000000;specifically for further use as hotmelt PSAs, PSAs having molecularweights (weight average M_(w)) of from 100000 to 800000, more preferablyfrom 100000 to 400000, g/mol are prepared. The polymerization may beconducted in the presence of an organic solvent, in the presence ofwater or in mixtures of organic solvents and water. The aim is tominimize the amount of solvent used. Depending on conversion andtemperature, the polymerization time is between 6 and 48 hours. Thehigher the reaction temperature which can be chosen, i.e., the higherthe thermal stability of the reaction mixture, the shorter the reactiontime that can be chosen.

For the purposes of the invention it is particularly advantageous tooperate the process such that the addition of the monomers possessingthe functional groups B and C and the reaction of the functional groupsB with the carboxylic anhydride take place directly after theconcentration step.

In one development of the invention, this operation takes place in anextruder: a twin-screw extruder (e.g. Werner & Pfleiderer, ZSK 40) or aco-kneader (e.g. Buss) have been found highly suitable for this purpose(reactive extrusion). In the extruder, the acrylic PSAs prepared byfree-radical polymerization are concentrated and freed from the solvent.Advantageously for the process of the invention, the solvent content ofthe polymer composition following the concentration operation is lessthan 0.5% by weight. Following concentration the component provided withthe functional groups B and C is added to the extruder or to theco-kneader, preferably by metering. Here, the reaction takes placebetween the functional groups B and the carboxylic anhydride groupsincorporated into the polymer chains. In one preferred embodiment of theinventive process, the addition may be made in a second extruder. Inthis case the optimum reaction conditions can be set by means of barrellength, throughput (rotary speed), kneading temperature, and amount ofany catalyst added. Moreover, a relatively low-shear screw geometry ofthe extruder should be chosen in order to prevent instances of gellingduring operation.

Advantageously for the purposes of the invention it is possible to useany compounds which contain crosslinkable groups C and which alsopossess a hydroxyl function that can react with the carboxylicanhydride.

As well as compounds substituted by hydroxyl groups, it is possible tomake favorable use, for the inventive process, of compounds containingcrosslinkable groups C and also containing other functional groups whichare able to react, directly or under catalysis, with the carboxylicanhydride, particularly in a linking reaction. Such functional groupsare familiar to the skilled worker; here, mention may be made, by way ofexample and without wishing to be unnecessarily

restricted by their listing, of the following: alkoxy groups, mercaptogroups, thioether groups, hydroxyl groups, unsubstituted and substitutedamino groups, oxazolines and/or unsubstituted or substituted amidogroups, as well as all other functional groups which are reactive withcarboxylic anhydrides in the sense outlined above.

In order to ensure good and efficient crosslinking of the polymers,examples of crosslinkable groups B used to outstanding effect are vinylgroups or, even more preferably, acrylate or methacrylate groups, whichmay also be in the form of their substituted derivatives. Accordingly,monomers containing functional groups B and C which are advantageous inthe sense of the inventive concept are hydroxyl-containing acrylates,such as, very preferably, for example, 2-hydroxyethyl acrylate (2-HEA,acrylic acid 2-hydroxyethyl ester), hydroxypropyl acrylate (acrylic acid3-hydroxypropyl ester), and hydroxyl-containing methacrylates, such as2-hydroxyethyl methacrylate (2-HEMA, methacrylic acid 2-hydroxyethylester), hydroxypropyl methacrylate (methacrylic acid 3-hydroxypropylester), for example, and/or vinyl compounds, such as 1-decenol, forexample, oxazolines, such as ricinene-alkyloxazoline orsoyaalkyloxazoline, for example, acrylamides, such asbutoxymethylacrylamide, for example, or substituted amino compounds,such as tert-butylaminoethyl methacrylate, for example.

The molar fraction of the compound added which is functionalized by thegroups B and C corresponds preferably to the molar amount of thecarboxylic anhydride which is incorporated by polymerization in thepolyacrylate chain, but may also be chosen to be smaller or larger thanthat amount.

The amount of the compound functionalized by the groups B and C which isadded is very preferably chosen such that the molar ratio of the numbern_(B) of functional groups B of the added monomers to the number n_(CSA)of the copolymerized carboxylic anhydride units n_(B)/n_(CSA), is withina range of magnitude of between 0.8 and 1.2, very preferably between 0.8and 1, i.e., n_(B)/n_(CSA) is ≦1.

In one preferred variant of the inventive process between 0.1 and 25%,preferably between 1 and 19%, by weight of the compounds functionalizedby groups B and C—based on the polymer—are added. Through the amount ofthe copolymerized carboxylic anhydride and through the amount of thecompound functionalized with B or C it is possible to control thereaction rate for the polymer-analogous reaction in the melt.

In one procedure which is very favorable for the process, catalysts areadded in order to raise the reactivity. The fraction of the catalyst isbetween 0.01 and 5 mol %, but preferably between 0.1 and 0.5 mol %,based on the carboxylic anhydride.

The reaction proceeds under catalysis by acid or bases. As acids it ispossible to use all Lewis acid compounds. The reaction proceedspreferably with p-toluenesulfonic acid, itaconic acid, dibutyltin oxideor with sodium acetate. As bases it is possible to use all Lewis bases.The reaction proceeds preferably under 4-vinylaniline catalysis.

In accordance with the flow viscosity of the polyacrylate used, thereaction proceeds at elevated temperatures. The temperatures chosen arepreferably between 60 and 180° C.: in one particularly preferred range,between 110 and 160° C.

For the process of the invention it may likewise be of advantage to varythe molecular weight and to improve the processing properties in themelt. Thus it is possible, for example, by reducing the molecular weightto lower the flow viscosity and so to increase the reaction propensity.A further point is the processing properties under shear in theextruder, since PSAs of relatively low viscosity and relatively lowmolecular mass are easier to process in the extruder and the shearintroduced is therefore greatly reduced.

Compounding—that is, the addition of further additives—may generally becarried out likewise in the same apparatus as the previous steps, in afurther extruder or in a compounder, where additional commixing of thepolymer composition may also take place.

To produce the adhesive tapes, the polymers described above areoptionally blended with crosslinkers: suitable crosslinker substances inthis sense are difunctional or polyfunctional acrylates, difunctional orpolyfunctional isocyanates or difunctional or polyfunctional epoxides.It is, however, also possible here to use any further difunctional orpolyfunctional compounds which are familiar to the skilled worker andare capable of crosslinking polyacrylates.

For crosslinking with ultraviolet radiation, photoinitiators are used.Examples of photoinitiators that may be mentioned, without wishing toimpose unnecessary restriction, include cleaving (radical-forming)photoinitiators, especially α-cleavers, and hydrogen abstractors. Forthe group of the photo cleaving initiators, examples that may bementioned include aromatic carbonyl compounds, especially benzoinderivatives, benzil ketals, and acetophenone derivatives. The hydrogenabstractors include, for example, aromatic ketones, such asbenzophenone, benzil, and thioxanthones, for example.

Moreover, in order to prepare pressure sensitive adhesives, theseelastomers are optionally blended with at least one resin. Tackifyingresins to be added include without exception all existing tackifierresins described in the literature. Representatives that may bementioned include pinene resins, indene resins, and rosins, theirdisproportionated, hydrogenated, polymerized, esterified derivatives andsalts, the aliphatic and aromatic hydrocarbon resins, terpene resins andterpene-phenolic resins, and also C5, C9 and other hydrocarbon resins.Any desired combinations of these and further resins may be used inorder to adjust the properties of the resulting adhesive in accordancewith what is desired. In general it is possible to use all resins whichare compatible (soluble) with the corresponding polyacrylate. Explicitreference is made to the depiction of the state of the art in the“Handbook of Pressure Sensitive Adhesive Technology” by Donatas Satas(van Nostrand, 1989).

The acrylic hotmelts may further be blended with one or more additivessuch as aging inhibitors, light stabilizers, ozone protectants, fattyacids, resins, plasticizers, nucleators, blowing agents, andaccelerators. As aging inhibitors it is possible to use both primary andsecondary aging inhibitors and also light stabilizers, including theircombination with one another. Reference is made only at this point tothe appropriate Irganox™ grades from Ciba Geigy and Hostanox™ fromClariant. As further outstanding agents against aging it is possible touse phenothiazine (carbon radical scavenger) and also hydroquinonemethyl ether in the presence of oxygen, and also oxygen itself.

Furthermore, the hotmelt PSAs may be filled with one or more fillerssuch as fibers, carbon black, zinc oxide, titanium dioxide, solidmicrobeads, solid or hollow glass beads, silica, silicates, and chalk,with the addition of blocking-free isocyanates being a furtherpossibility.

The acrylic PSAs blended in this way are preferably processed furtherfrom the melt (as hotmelts). For use as an adhesive for adhesive tapes,they are coated onto a backing and then crosslinked in order to increasethe cohesion.

It is advantageous to carry out coating of the functionalized acrylicPSA from the melt in gel-free form. For this purpose it is preferred touse melt dies or extrusion dies having a slot width of from 100 to 500μm, more preferably from 150 to 300 μm.

As backing materials in this context it is possible to use the materialswhich are customary and familiar to the skilled worker, such as films(polyesters, PET, PE, PP, BOPP, PVC), nonwovens, foams, wovens and wovenfilms, and also, where appropriate, release paper (for example,glassine, HDPE, LDPE). This list is not intended to be exclusive.

The adhesives are crosslinked with UV light or with ionizing radiation.Remaining vinyl compounds which have not undergone reaction during thehotmelt process or reactive extrusion react with the radicals that areformed during crosslinking and at this point in time, at the latest,become attached to the polymer, so that they are no longer able toescape from the PSA tape at a later time. Crosslinking by UV rays orelectron beams proceeds with great efficiency owing to the double bondsthe side chains contain.

In comparison to non-functionalized polyacrylates, it is possible tolower the dose required for optimum crosslinking, thereby requiring lessenergy and, in the case of electron beam crosslinking, causing lessdamage to the backing material. Moreover, a cohesion-enhancing effecthas been obtained.

In contrast to polyacrylates modified by allylic double bonds, there isno substantial reduction in thermal stability in the case of thepolyacrylates produced by the inventive process. The thermal stabilityremains sufficiently high for processing by the hotmelt coating process.Accordingly, all-acrylate systems prepared in this way are gel-free forat least 48 hours at 140° C., resin-blended systems at 120° C.

The process of the invention opens up application of the process ofreactive extrusion for the preparation of polyacrylate-based PSAs. Thisresult is surprising and could not have been foreseen by the skilledworker: on the contrary, the skilled worker would have expected the verydrastic operating conditions (high temperatures, long residence times)typical of reactive extrusion to lead to a high level of gelling in theextruder.

Accordingly, polyacrylates prepared in the process of the invention haveincorporated into them carboxylic anhydride groups, carboxylic acidgroups, and hydroxyl groups; furthermore, (meth)acrylate groups arepresent as side chains. Under the conditions of reactive extrusionsecondary reactions would have been expected, in the form for example oftransesterification reactions, particularly those of the polymer chainswith one another. Secondary reactions of this kind would result in ahigh level of gelling of the polyacrylate composition. Unexpectedly,such reactions were to all intents and purposes not observed; instead, areaction takes place preferentially, in accordance with the invention,between the carboxylic acid groups (preferably maleic anhydride groups)and the functional groups B of the added monomers (preferably hydroxylgroups). This, surprisingly, allows polymer-analogous reactions to becarried out in an extruder, leading to low extruder residence timesowing to the high reaction rates.

In this system, gel-free polyacrylate compositions can be prepared whichhave a high stability in respect of a gelling process (“gel-free”indicates compliance with the requirements for coatability of thecompositions using the standard coating apparatus). Owing to the freedomfrom gel, the polyacrylate compositions can be used for adhesives whichcan be coated from the melt, and can thus be used as PSAs for PSA tapes,for example. The coatability is distinguished by a uniform (homogeneous)coating pattern, with no inhomogeneities, if coating takes place throughthe standard coating dies (melt dies or extrusion dies having a slotwidth of from 100 to 500 μm, more preferably from 150 to 300 μm) onto,for example, polyester backings with a thickness of 50 μm. Thepolyacrylate compositions commonly prepared in reactive extrusionprocesses do not meet these requirements and cannot be used as PSAs. Thecoating of the PSA onto a backing takes place very preferably in aninline process, though as an alternative can also be operated offline.After coating onto the backing, the PSA can then be subjected to thedesired crosslinking reaction.

The process of the invention provides for the first time theincorporation of (meth)acrylate groups into the side chains ofpolyacrylates in hotmelt systems. This presents the advantage of verygentle crosslinking methods, since crosslinking can be carried outdirectly by way of the incorporated acrylate groups. Where crosslinkingis carried out using electron beams, the crosslinking reaction rate isvery high and the conversion of the acrylate groups is high.Accordingly, polyacrylate PSAs prepared and crosslinked by the inventiveprocess possess very little, if any, post-crosslinking potential.Crosslinker substances which are normally added in addition aregenerally not reacted fully during the crosslinking reaction, with theconsequences that the PSAs age and the PSA products become unusable overthe course of time.

EXAMPLES

Commercially Available Chemicals Used—Trade Names

Product Manufacturer Chemical composition Novares Rüttgers Aliphatic,modified hydrocarbon resin T K90 comprising a copolymer of unsaturatedaromatic C9/C10 hydrocarbons and an aliphatically unsaturated componentSoftening range 85 to 95° C. Vazo 67 DuPont2,2′-Azobis(2-methylbutyronitrile) Perkadox 16 Akzo NobelBis-(4-tert-butylcyclohexyl) peroxydicarbonate Irgacure 819 Ciba GeigyBis(2,4,6-trimethylbenzoyl)- phenylphosphine oxideTest Methods

The following test methods were used to evaluate the technical adhesiveproperties of the PSAs prepared.

Shear Strength (Test A)

A strip 13 mm wide of the adhesive tape was applied to a smooth steelsurface which had been cleaned three times with acetone and once withisopropanol. The area of application measured 20 mm×13 mm(length×width). The adhesive tape was then pressed onto the steelbacking four times using a 2 kg weight. At room temperature, a 1 kgweight was fastened to the adhesive tape and the time taken for theweight to fall off was measured.

The shear stability times measured are reported in minutes andcorrespond to the average of three measurements.

Determination of the Gel Fraction (Test B)

The carefully dried, solvent-free adhesive samples are welded into apouch of polyethylene nonwoven (Tyvek web). From the difference in thesample weights before and after extraction with toluene the gel index isdetermined, i.e., the percentage weight fraction of the polymer that isnot soluble in toluene.

IR Spectroscopy

The FT-IR IFS 45 spectrometer from Bruker was used for the measurement.A calibration plot was first compiled using different concentrations ofthe individual carboxylic anhydrides. The conversion of thecorresponding fractions of carboxylic anhydride was determined bymeasuring the percentage fall in the CO band.

Samples Analyzed

The carboxylic anhydrides used are available commercially. 2-HEA(2-hydroxyethyl acrylate) and 2-HEMA (2-hydroxyethyl methacrylate) werepurified by distillation beforehand and stored under a nitrogenatmosphere.

Example 1

A reactor conventional for free-radical polymerizations was charged with500 g of 2-ethylhexyl acrylate, 350 g of methyl acrylate, 70 g of butylacrylate, 80 g of 4-methacryloyloxyethyl trimellitate anhydride and 540g of acetone/special-boiling-point spirit (1:1). Nitrogen gas was passedthrough the mixture for 45 minutes, followed by double degassing, afterwhich the reactor was heated to 58° C. with stirring and 0.2 g ofazoisobutyronitrile (AIBN) was added. The external heating bath was thenheated to 70° C. and reaction was carried out constantly at thisexternal temperature. After a reaction time of 1 hour a further 0.2 g ofAIBN was added. After 3 hours and 6 hours, in each case 250 g ofacetone/special-boiling-point spirit (1:1) were used for dilution. Thereaction was terminated after a time of 24 hours and the mixture wascooled to room temperature.

For adhesive testing, 100 g of the adhesive (based on solids) wereblended with 0.4 g of bis(2,4,6-trimethylbenzoyl)phenylphosphine oxide(Irgacure 819; Ciba Geigy) and the adhesive was applied at a coverage of50 g/m² (based on solids) to a primed PET film (23 μm thick). Thespecimens were then irradiated at 20 m/min (4 passes under the lamp)using a UV irradiation unit from Eltosch (254 nm, 120 W/cm).

The resulting specimens were then subjected to adhesive testing inaccordance with test methods A and B.

Example 2

A reactor conventional for free-radical polymerizations was charged with10 g of acrylic acid, 375 g of 2-ethylhexyl acrylate, 200 g of methylacrylate, 375 g of butyl acrylate, 40 g of itaconic anhydride and 290 gof acetone/special-boiling-point spirit (1:1). Nitrogen gas was passedthrough the mixture for 45 minutes, followed by double degassing, afterwhich the reactor was heated to 58° C. with stirring and 0.2 g ofazoisobutyronitrile (AIBN) was added. The external heating bath was thenheated to 75° C. and reaction was carried out constantly at thisexternal temperature. After a reaction time of 1 hour a further 0.2 g ofAIBN was added. After 3 hours and 6 hours, in each case 250 g ofacetone/special-boiling-point spirit (1:1) were used for dilution. Thereaction was terminated after a time of 24 hours and the mixture wascooled to room temperature.

For adhesive testing, the adhesive was applied at a coverage of 50 g/m²(based on solids) to a primed PET film (23 μm thick). The specimens werethen irradiated with an electron beam dose of 20 kGy at an accelerationvoltage of 230 kV (EBC unit from Crosslinking). The resulting specimenswere then subjected to adhesive testing in accordance with test methodsA and B.

Example 3

A reactor conventional for free-radical polymerizations was charged with20 g of acrylic acid, 810 g of 2-ethylhexyl acrylate, 50 g of methylacrylate, 120 g of 4-methacryloyloxyethyl trimellitate anhydride and 540g of acetone/special-boiling-point spirit (1:1). Nitrogen gas was passedthrough the mixture for 45 minutes, followed by double degassing, afterwhich the reactor was heated to 58° C. with stirring and 0.2 g ofazoisobutyronitrile (AIBN) was added. The external heating bath was thenheated to 70° C. and reaction was carried out constantly at thisexternal temperature. After a reaction time of 1 hour a further 0.2 g ofAIBN was added. After 3 hours and 6 hours, in each case 250 g ofacetone/special-boiling-point spirit (1:1) were used for dilution. Thereaction was terminated after a time of 24 hours and the mixture wascooled to room temperature.

For adhesive testing, the adhesive was applied at a coverage of 50 g/m²(based on solids) to a primed PET film (23 μm thick). The specimens werethen irradiated with an electron beam dose of 15 kGy at an accelerationvoltage of 230 kV (EBC unit from Crosslinking). The resulting specimenswere then subjected to adhesive testing in accordance with test methodsA and B.

Example 4

A reactor conventional for free-radical polymerizations was charged with20 g of acrylic acid, 430 g of 2-ethylhexyl acrylate, 100 g of methylacrylate, 430 g of butyl acrylate, 20 g of maleic anhydride and 212 g ofacetone/special-boiling-point spirit (1:1). Nitrogen gas was passedthrough the mixture for 45 minutes, followed by double degassing, afterwhich the reactor was heated to 58° C. with stirring and 0.2 g ofazoisobutyronitrile (AIBN) was added. The external heating bath was thenheated to 75° C. and reaction was carried out constantly at thisexternal temperature. After a reaction time of 1 hour a further 0.2 g ofAIBN was added. After 3 hours and 6 hours, in each case 300 g ofacetone/special-boiling-point spirit (1:1) were used for dilution. Thereaction was terminated after a time of 24 hours and the mixture wascooled to room temperature.

For adhesive testing, the adhesive was applied at a coverage of 50 g/m²(based on solids) to a primed PET film (23 μm thick). The specimens werethen irradiated with an electron beam dose of 25 kGy at an accelerationvoltage of 230 kV (EBC unit from Crosslinking).

The resulting specimens were then subjected to adhesive testing inaccordance with test methods A and B.

Example 5

A reactor conventional for free-radical polymerizations was charged with1500 g of 2-ethylhexyl acrylate, 200 g of methyl acrylate, 100 g ofacrylic acid, 100 g of maleic anhydride, 100 g of N-tert-butylacrylamideand 330 g of acetone. Nitrogen gas was passed through the mixture for 45minutes, followed by double degassing, after which the reactor washeated to 66° C. with stirring and 1 g of Vazo 67™ (DuPont) was added.After 8 hours there was again addition of 1 g of Vazo 67™ (DuPont) and500 g of acetone. After 24 hours and 28 hours, in each case 2.5 g ofPerkadox 16 (Akzo Nobel) were added. After 32 hours, dilution wascarried out using 600 g of acetone. The reaction was terminated after 48hours and the mixture was cooled to room temperature.

For adhesive testing, the adhesive was applied at a coverage of 50 g/m²(based on solids) to a primed PET film (23 μm thick). The specimens werethen irradiated with an electron beam dose of 10 kGy at an accelerationvoltage of 230 kV (EBC unit from Crosslinking).

The resulting specimens were then subjected to adhesive testing inaccordance with test methods A and B.

Example 6

In comparison to Example 5, the acrylic PSA was blended with 30% byweight (based on the polymer) of hydrocarbon resin TK 90™ (Rüttgers) andused for coating. The procedure was as in Example 5. The composition wasirradiated with an EB dose of 30 kGy with an acceleration voltage of 230kV.

Implementation of the Hotmelt Operation in a Recording Extruder:

The shearing and thermal exposure of the acrylic hotmelts was carriedout using the Rheomix 610p recording extruder from Haake. The drive unitused was the Rheocord RC 300p device. The instrument was controlledusing the PolyLab System software. The extruder was charged in each casewith 52 g of the acrylic PSA/monomer mixture (˜80% fill level). Theexperiments were conducted at a kneading temperature of 110 or 130° C.,a rotary speed of 30 rpm, and a kneading time of one hour. The specimenswere subsequently coated as hotmelts through a slot die at about 130° C.

Example 1#

In analogy to Example 1, the acrylic PSA was freed from the solventafter cooling and 100 g of the acrylic hotmelt were mixed with 1.8 g of2-HEA (2-hydroxyethyl acrylate) and about 0.1 g of 4-vinylaniline. 52 gof this mixture were processed at 110° C. in the recording extruder inaccordance with the procedure set out above. After the end of thereaction and after coating, the procedure of Example 1 was followed.

The conversion in the reaction was measured by way of IR spectroscopy.

Example 2#

In analogy to Example 2, the acrylic PSA was freed from the solventafter cooling and 100 g of the acrylic hotmelt were mixed with 4.6 g of2-HEMA (2-hydroxyethyl methacrylate) and about 0.1 g of 4-vinylaniline.52 g of this mixture were processed at 130° C. in the recording extruderin accordance with the procedure set out above. After the end of thereaction and after coating, the procedure of Example 2 was followed. Theconversion in the reaction was measured by way of IR spectroscopy.

Example 3#

In analogy to Example 3, the acrylic PSA was freed from the solventafter cooling and 100 g of the acrylic hotmelt were mixed with 6.1 g of2-HEMA (2-hydroxyethyl methacrylate) and about 0.1 g of 4-vinylaniline.52 g of this mixture were processed at 130° C. in the recording extruderin accordance with the procedure set out above. After the end of thereaction and after coating, the procedure of Example 3 was followed.

The conversion in the reaction was measured by way of IR spectroscopy.

Example 4#

In analogy to Example 4, the acrylic PSA was freed from the solventafter cooling and 100 g of the acrylic hotmelt were mixed with 2.6 g of2-HEMA (2-hydroxyethyl methacrylate) and about 0.1 g of 4-vinylaniline.52 g of this mixture were processed at 130° C. in the recording extruderin accordance with the procedure set out above. After the end of thereaction and after coating, the procedure of Example 4 was followed.

The conversion in the reaction was measured by way of IR spectroscopy.

Example 5#

In analogy to Example 5, the acrylic PSA was freed from the solventafter cooling and 100 g of the acrylic hotmelt were mixed with 5.8 g of2-HEMA (2-hydroxyethyl methacrylate). 52 g of this mixture wereprocessed at 150° C. for 1 minute at 70 rpm in the recording extruder inaccordance with the procedure set out above. After the end of thereaction and after coating, the procedure of Example 5 was followed.

The conversion in the reaction was measured by way of IR spectroscopy.

Example 6#

In comparison to Example 5#, the functionalized acrylic hotmelt wasblended with 30% by weight (based on the polymer) of hydrocarbon resinTK 90™ (Rüttgers) and coated as a hotmelt from the melt. The procedurewas as in Example 5#. The composition was irradiated with an EB dose of30 kGy with an acceleration voltage of 230 kV.

Results

To prepare the acrylic pressure sensitive adhesives, the followingacrylates were first of all polymerized with the comonomerconcentrations compiled in Table 1. Polymerization was carried outconventionally using AIBN in a mixture of acetone andspecial-boiling-point spirit. The individual reaction regimes have beendescribed in the section above.

TABLE 1 Example AS [%] 2-EHA [%] MA [%] n-BA [%] anhydride 1 0 50 35 7 8% BSI 2 1 37.5 20 37.5  4% ISA 3 2 81 5 0 12% BSI 4 2 43 10 43  2% MSA5 5 75 10 0 15% MSA AS: acrylic acid; 2-EHA: 2-ethylhexyl acrylate; MA:methyl acrylate; n-BA: n-butyl acrylate; MSA: maleic anhydride; ISA:itaconic anhydride; BSI: 4-methacryloyloxyethyl trimellitate anhydride.

In addition to their use for reactive extrusion, examples 1-5 were alsosubjected to adhesive testing and used as references. For this purposethe polymers were applied conventionally from solution onto a primedpolyester film 23 μm thick. After drying at 120° C. for 10 minutes theapplied mass of the pure adhesive was 50 g/m². After curing, the gelindex of the specimens irradiated with electron beams or UV light wasmeasured, after which the cohesion was determined by way of the sheartest at room temperature. Table 2 shows the results.

TABLE 2 Electron Shear stability beam dose UV^(a) lamp Gel index timesExample [kGy] passage [%] RT, 10 N [min] 1 0 4 x^(b) 58 580 2 20 0 391490 3 15 0 30 7785 4 25 0 42 6840 5 10 0 5 5810 6 30 0 2 165 ^(a)120W/cm, 254 nm wavelength (medium pressure rotary lamp from Eltosch), 20m/min web speed; ^(b)with 0.4% by weight ofbis(2,4,6-trimethylbenzoyl)-phenylphosphine oxide. Application coverage:50 g/m²

Example 1 was cured with UV light (254 nm). At a web speed of 20 m/min agel index of 58% was obtained. As a result of the relatively high apolar fraction, the cohesion of this adhesive is low. In contrast,examples 2-6 were cured with electron beams. The gel indices measuredlay between 2 and 42%. The shear strength as well, with a shearingweight of 10 N, was in all cases clearly below the required mark of10000 minutes for an acrylic PSA of high shear strength. Example 6 lentitself particularly poorly to crosslinking by means of electron beams,since electron beam crosslinking in the presence of resins is generallyless efficient.

With these results as reference, Examples 1-5 were concentrated, i.e.,freed from solvent, and so prepared for the hotmelt operation. Theacrylic hotmelts were then reacted optionally with 0.1% by weight of4-vinylaniline and with different amounts of hydroxylated acrylates. Thereaction was conducted in a recording extruder, which is able to varythe degree of shear and the reaction temperature. It is also possible torecord the variation in torque. For clarity, the process parameters andthe amounts of vinyl compounds used are listed in Table 3.

TABLE 3 % by wt. of 4- % by % by Rotary Reaction vinyl- wt. of wt. ofspeed temperature Example aniline 2-HEA 2-HEMA [rpm] [° C.] 1# 0.1 1.8 030 110 2# 0.1 0 4.6 30 130 3# 0.1 0 6.1 30 130 4# 0.1 0 2.7 30 130 5# 00 5.8 70 150 2-HEA = 2-hydroxyethyl acrylate; 2-HEMA = 2-hydroxy-ethylmethacrylate

Example 1# was reacted with only 0.5 mole equivalent of 2-HEA. Incontrast, examples 2#-4# were admixed in each case with equimolaramounts of 2-HEMA. The catalyst selected was 4-vinylaniline, since thedouble bond is incorporated into the polyacrylate during crosslinkingand so no residual fractions of base remain as volatile fractions in thePSA. The rotary speed for mixing was 30 rpm and so was relatively low,in order to simulate low shear of the PSA. In the case of Example 5#, ahigher shear was introduced. The reaction temperature was set at 110° C.for Example 1#, since this polymer was the lowest in viscosity. Forexamples 2#-4# the reaction and extrusion temperature was 130° C., inthe case of Example 5# it was 150° C. The reaction time for allspecimens lay at 1 hour, in the case of Example 5# at 1 minute.Thereafter, the examples were firstly coated as hotmelts through a dieonto a primed polyester backing (23 μm thick) and then, depending onexample, cured in an analogy to Table 2 using UV or electron beams andsubjected to adhesive testing. The application coverage of the pureacrylic PSA was again 50 g/m². In addition, the conversion in thereaction was determined by FT-IR. The results of these tests aresummarized in Table 4.

TABLE 4 Shear Electron stability Conversion beam UV^(a) Gel times ofdose lamp index RT, 10 N anhydride Example [kGy] passage [%] [min] [%]1#  0 4 x^(b) 70  4 675 38 2# 20 0 64 +10 000 80 3# 15 0 68 +10 000 764# 25 0 52 +10 000 82 5# 10 0 65 +10 000 84 6# 30 0 41  3 415 84 ^(a)120W/cm, 254 nm wavelength (medium pressure rotary lamp from Eltosch), 20m/min web speed; ^(b)with 0.4% by weight ofbis(2,4,6-trimethylbenzoyl)-phenylphosphine oxide. Application coverage:50 g/m²

The conversion of anhydride, expressed by the percentage decrease in theCO IR band, is relatively low in the case of Example 1#, since only 0.5mole equivalent of 2-HEA was used. Nevertheless, the effect wasconsiderable for UV crosslinking. Under the same crosslinkingconditions, the gel index rose from 58 to 70%. In addition, thecomposition became significantly more cohesive after crosslinking, owingto the reactive extrusion with 2-HEA. The same trend was recorded forexamples 2#-5#. The gel indices rose, in some cases considerably, andthe shear strength of these examples was generally more than 10000minutes. Here, a conversion of about 80% was detected by IRspectroscopy. Even the sample blended with resin shows that by reactiveextrusion it is possible to achieve a significant reduction in theelectron beam dose required.

With these examples it can be demonstrated that the principle ofreactive extrusion can be utilized for the preparation of readilycrosslinkable acrylic hotmelts. Furthermore, a cohesion-enhancing effectwas found.

As a result of the marked increased crosslinking rate it is possible toachieve much better and more effective crosslinking, especially ifacrylic and methacrylic double bonds have been introduced ascrosslinking-functional groups. Even acrylic PSAs prepared by thisprocess which have a molecular weight reduced by 20-40% as compared withconventionally prepared (not functionalized) acrylic PSAs achieveadhesive properties which are just as good as those of theconventionally prepared PSAs of higher molecular weight, while owing tothe low molecular weight they have a greatly reduced viscosity andtherefore a considerably improved processing quality in the hotmeltoperation.

1. A process for preparing crosslinked acrylic pressure sensitiveadhesives which comprises first preparing polyacrylates by free-radical(co)polymerization in water, organic solvent or solvents or a mixturethereof from a monomer mixture comprising: (a) 45-95% by weight ofacrylic acid monomers, methacrylic acid monomers, or both, of thefollowing structure:

where R₁=H or CH₃ and R₂=an alkyl chain with 2-20 carbon atoms, (b) 0.5to 25% by weight of one or more carboxylic anhydrides containingolefinic double bonds, (c) 0-30% by weight of further olefinicallyunsaturated monomers possessing functional groups A, concentrating thepolymers thus prepared to form a concentrated polyacrylate compositionhaving a water, solvent or water and solvent concentration of ≦2% byweight, adding to the concentrated polyacrylate composition furthermonomers which have at least two functional groups B and C, the groups Bbeing groups which are capable of entering into polymer-analogousreactions with the carboxylic anhydrides, and the groups C beingcrosslinkable groups, reacting the functional groups B with thecarboxylic anhydride, to attach the monomers containing functionalgroups B as side chains to the polymers, after the reaction between thefunctional groups B and the carboxylic anhydride, crosslinking thepolymers with high-energy radiation to form a cross-linked acrylicpressure sensitive adhesive.
 2. The process of claim 1, wherein afterreaction of the functional groups B with the carboxylic anhydride andbefore the crosslinking step, the composition is applied from the meltto a backing.
 3. The process of claim 1, wherein the addition of themonomers possessing the functional groups B and C and the reaction ofthe functional groups B with the carboxylic anhydride take placedirectly after the concentration step.
 4. The process of claim 1,wherein the solvent, water or solvent and water content after theconcentration step is ≦0.5% by weight.
 5. The process of claim 1,wherein the functional groups B of the monomers are selected from thegroup consisting of hydroxyl groups, alkoxy groups, mercapto groups,thioether groups, unsubstituted and substituted amino groups, oxalines,unsubstituted or substituted amido groups and combinations thereof. 6.The process of claim 1, wherein the functional groups C of the monomersare selected from the group consisting of vinyl groups, acrylate groups,methacrylate groups and combinations thereof.
 7. The process of claim 1,wherein the monomers containing functional groups B and C are selectedfrom the group consisting of 2-hydroxyethyl acrylate (2-HEA, acrylicacid 2-hydroxyethyl ester), hydroxypropyl acrylate (acrylic acid3-hydroxypropyl ester), 2-hydroxyethyl methacrylate (2-HEMA, methacrylicacid 2-hydroxyethyl ester), hydroxypropyl methacrylate (methacrylic acid3-hydroxypropyl ester) and combinations thereof.
 8. The process of claim1, wherein the molar ratio of the number n_(s) of the functional groupsB of the added monomers to the number n_(CSA) of the copolymerizedcarboxylic anhydride units, n_(B)/n_(CSA), is between 0.8 and 1.2. 9.The process of claim 1, wherein a catalyst is added to the polyacrylatecomposition.
 10. The process of claim 1, wherein the polymer-analogousreaction is conducted at a temperature of between 60° C. and 180° C. 11.The process of claim 1, wherein resins or additives selected from thegroup consisting of aging inhibitors, light stabilizers, ozoneprotectants, fatty acids, plasticizers, nucleators, blowing agents,accelerators, fillers and combinations thereof are added to the monomermixture or to the acrylic pressure sensitive adhesive.
 12. The processof claim 1, wherein crosslinkers, photoinitiators or both are added tothe polymeric composition to be crosslinked.
 13. The process of claim 1,wherein electron beams or UV radiation are used as high-energy radiationfor the crosslinking.
 14. An adhesive tape comprising the acrylicpressure sensitive adhesive prepared by the process of claim 1 on one orboth sides of a backing.
 15. The process of claim 3, wherein saidreaction is conducted in an extruder.
 16. The process of claim 8,wherein said ratio is between 0.8 and
 1. 17. The process of claim 9,wherein said catalyst is a Lewis acid or a Lewis base.
 18. The processof claim 17, wherein said Lewis acid is p-toluenesulfonic acid and saidLewis base is 4-vinylaniline.
 19. The process of claim 10 wherein saidtemperature is between 110°and 160° C.
 20. The process of claim 12,wherein said crosslinkers are selected from the group consisting ofdifunctional acrylates, difunctional methacrylates, polyfunctionalacrylates, polyfunctional methacrylates and combinations thereof.